Method for measuring the speed of an induction machine

ABSTRACT

In order to measure the rotation speed of an induction machine whose stator is connected via a controllable AC controller to a single-phase or polyphase AC power supply system, the stator is disconnected from the AC power supply system for at least a predetermined time period (Δt). This is preferably achieved by opening active devices in the AC controller, with at least one stator voltage, which is induced in the stator by the rotary movement of the rotor, being measured in this time period (Δt). The measured values are used to determine the frequency of the stator voltage and to derive the rotation speed of the induction machine.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/DE99/02876 which has an Internationalfiling date of Sep. 10, 1999, which designated the United States ofAmerica.

FIELD OF THE INVENTION

The invention relates to a method for measuring the rotation speed of aninduction machine whose stator is connected via a controllable ACcontroller to a single-phase or polyphase AC power supply system. Theinvention also relates to a device for determining the rotation speed ofan induction machine.

BACKGROUND OF THE INVENTION

It is known for controllable AC controllers to be used for matching theelectric volt-amperes supplied to an induction machine to therespectively prevailing load conditions, in particular during startingand braking.

Such a microprocessor-controlled AC controller or soft starter, as isknown for example from EP 0 454 697 B1, operates using the phase-gatingprinciple and is used essentially for smooth starting and stopping ofthree-phase asynchronous machines. Three sets of active devices, ingeneral each including two back-to-back connected thyristors, aregenerally actuated by a microprocessor for this purpose.

The control device in the known three-phase controller has noinformation about the present rotation speed of the machine. Withcertain mechanical load conditions, this can lead to poor operation ofthe overall drive. When stopping a pump drive, an abrupt drop inrotation speed can occur, which can lead to extremely high pressures inthe pipeline system and thus to severe mechanical loads, and even todestruction of the system. A corresponding situation applies to thestarting of drives when a sudden rise in the rotation speed occurs.

If the rotation speed is known, the control device, generally amicroprocessor, can be used to provide rotation speed control whichallows largely smooth starting and stopping of the drive, even when themechanical load conditions are poor.

DE 27 15 935 A1 discloses a starting monitor for asynchronous machines,in which the phase angle between the current and voltage is determined.This is used to derive binary information about the starting of themachine. If starting does not take place within a specific time period,the machine is disconnected from the power system once again in order toavoid thermal overloads.

In U.S. Pat. No. 5,548,197A, the current zero crossings of the threestator currents are detected using, inter alia, the voltage which can bemeasured across the thyristors for this purpose. Two immediatelysuccessive current zero crossings are used to form an error signal bysubtracting the times of the zero crossings from one another and thensubtracting one-sixth of the power supply system period. The errorsignal, which fluctuates about the zero point, is subjected to frequencyanalysis, and the rotation speed of the rotor is determined from this.Power supply system disturbances can in this case result in corruptionof the measurement signal.

A method which measures the polarity of the induced terminal voltageduring the process of stopping an induction machine by means of athree-phase controller and which determines the rotation speed from thetime difference between the polarity changes of the individual voltagesis described in EP 0 512 372 B1. In any case, during the stoppingprocess, there are time periods in which the induction machine isdisconnected from the power supply system and in which, in consequence,no currents flow in the stator either. There is thus no need tointerrupt the current supply solely to measure the rotation speed. Inthis case, in order to brake the induction machine, specific triggersequences of thyristors are defined in advance, and the time offset ofthe respective polarity change is evaluated as the frequency fordetermining the rotation speed. On the other hand, no method isspecified for general starting and stopping.

U.S. Pat. No. 5,644,205A and DE 195 03 658 C3 each indicate a method formeasuring the rotor angular velocity for machines usingfrequency-changing control. These methods use the frequency of theinduced voltage once the power supply has been disconnected from themachine to determine the rotor angular velocity. Owing to theconsiderably different functional principles of frequency changers andthree-phase controllers, the method of producing a stator withoutcurrent, which is known from the cited documents, cannot be transferredto machines controlled by three-phase controllers.

SUMMARY OF THE INVENTION

Against the background of the prior art, the invention is now based onthe object of specifying a method for measuring the rotation speed of aninduction machine, which can be carried out easily during accelerationduring the starting of the induction machine and in which there is noneed for any additional measured value sensors for detecting therotation speed. Furthermore, the invention is based on the object ofspecifying a device for controlling such an induction machine.

According to the invention, the first-mentioned object is achieved by amethod for measuring rotation speed of an induction machine whose statoris connected, via a controllable AC controller having active devicearrangements, to an AC power supply. The method includes controlling theactive device arrangements to disconnect the stator from the AC powersupply system for at least one predetermined time period (Δt), which isless than half of a period (T) of a voltage of the AC power supplysystem, by controlling the active device arrangements; measuring in thetime period (Δt), a voltage which is induced in the stator by rotarymovement of a rotor and using the measured voltage to determinecomponents of a stator voltage space vector; and determining a rotationfrequency of the stator voltage space vector from the measured voltage,and deriving the rotation speed of the induction machine therefrom. Inthe method for measuring the rotation speed of an induction machinewhose stator is connected via a controllable AC controller to asingle-phase or polyphase AC power supply system, the stator isdisconnected from the AC power supply system for at least apredetermined time period by controlling the active devices in the ACcontroller. At least one stator voltage, which is induced in the statorby the rotary movement of the rotor, is measured in this time period.The measured values obtained in this way are used to determine thefrequency of this stator voltage, and the rotation speed of theinduction machine is derived from this.

The stator is thus temporarily placed in a situation where no current isflowing during acceleration of the induction machine. During the timeperiod in which no stator current is flowing, a slowly decaying directcurrent flows in the rotor. As a result of this, the rotor can beregarded as a rotating magnet with virtually constant magnetic flux,with respect to the rotor coordinate system. The rotation inducesvoltages (terminal voltages) across the stator terminals of theinduction machine, whose frequency corresponds to the product of theknown number of pole pairs p and the mechanical rotation speed to bemeasured.

According to the invention, the rotation speed of the rotor is detectedusing the frequency of the stator voltage space vector, which can bedetermined on the basis of the induced voltage, during a time periodwhich is produced deliberately with the aid of the controllable ACcontroller and in which no current flows in the stator. According to theinvention, the time duration of this time period is shorter than thetime duration of half the period of the power supply system voltage, inorder to influence the operation of the drive only to a minor extent.

For the same reasons, in a further preferred refinement of theinvention, the rotation speed measurement in accordance with theabovementioned method is repeated after specific time periods, which arepreferably 5 to 15 times the period of the power supply system voltage.

Thyristors are preferably used as the active device arrangements, andthe induction machine is disconnected from the AC power supply system byomitting the trigger signals required to trigger the thyristors.

In one particularly preferred refinement of the invention, in order toresume the power supply system operation of the induction machine in thecase of a three-phase induction machine, the first trigger signal forthe first-opened first active device arrangement in one phase is delayedby a multiple of half the power supply system period with respect to thelast trigger signal for this first active device arrangement. At thesame time as the renewed triggering of this first active devicearrangement, a second active device arrangement is triggered, which isan active device arrangement that is triggered subsequently in normaloperation. The third active device arrangement is triggered one-sixth ofthe power supply system period after the triggering of the first activedevice arrangement, with the trigger signal sequence which was presentbefore the disconnection then being reproduced. This measure ensuresthat the interruption in the voltage supply to the induction machinewhich follows the rotation speed measurement has as little influence aspossible on the continued operation of the induction machine.

In a further advantageous refinement of the method, during the timeperiod during which no current is flowing in the stator in the case of apolyphase AC power supply system, the terminal voltages which are ineach case induced in the stator windings between the stator terminalsare measured. The angle of the space vector of the induced statorvoltage is in each case calculated, in particular, from the measuredvalues of the terminal voltage.

For discrete-time sampling of the induced terminal voltage, the clockrate is in this case defined such that the associated angles of thespace vector of the induced stator voltage are calculated for as manytimes as possible within the time period. The determined angles of thespace vector are associated, within the time period during which nocurrent is flowing in the stator, with a straight line from whosegradient the rotation speed of the induction machine is determined.

According to the invention, the second-mentioned object is achieved by adevice for determining rotation speed of an induction machine whosestator is connected via an AC controller to an AC power supply system.The device includes a control device for controlling the AC controller,and for disconnecting the stator from the AC power supply system for apredetermined time period (Δt), which is shorter than half a period (T)of a voltage of the AC power supply system, by controlling active devicearrangements of the AC controller; a voltage measurement device formeasuring at least one terminal voltage which is induced in the statorby rotary movement of a rotor in the time period (Δt); and a computationdevice for calculating a frequency of the measured terminal voltage andfor calculating the rotation speed of the induction machine from thecalculated frequency, wherein a control signal for the control device ispresent at one output of the computation device, the control signalbeing derived from the rotation speed and being passed to the controldevice. The device for controlling an induction machine, whose stator isconnected via an AC controller to a single-phase or polyphase AC powersupply system, contains a control device for controlling the ACcontroller and for disconnecting the stator from the AC power supplysystem for a predetermined time period by opening the active devicearrangements in the AC controller. It further includes a voltagemeasurement device for measuring at least one stator voltage which isinduced in the stator by the rotary movement of the rotor in this timeperiod. Finally, a computation device is included, for calculating thefrequency of this stator voltage from the measured values obtained inthis way, and for calculating the rotation speed of the inductionmachine from this frequency.

In one preferred embodiment, the rotation speed is used to derive acontrol signal for the control device. This control signal is producedat one output of the computation device and is passed via a control lineto the control device.

Further preferred embodiments of the device are evident from thesubsequent description of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to explain the invention further, reference is made to theexemplary embodiment in the drawing, in which:

FIG. 1 shows a device according to the invention for controlling athree-phase induction machine, illustrated in the form of a schematicblock diagram.

FIG. 2 shows the currents flowing in the stator windings, plotted in theform of a graph with respect to time.

FIG. 3 shows the terminal voltages measured between each of theterminals of the stator, likewise plotted in the form of a graph withrespect to time.

FIG. 4 shows the time profile of the angle of the space vector of theinduced voltage, likewise in the form of a graph.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to FIG. 1, an induction machine 2, in the example of athree-phase asynchronous machine, is connected via a three-phase ACcontroller 4 (three-phase controller) to the phases L1, L2, L3 of athree-phase power supply system. Each phase L1, L2, L3 has an associatedactive device arrangement V1, V2, V3 which, in the exemplary embodimentas shown in FIG. 1, each include two back-to-back parallel connectedthyristors 6. The triggering electrodes of the thyristors 6 areconnected to a control device 8, which produces the trigger signalsrequired to trigger the thyristors 6, in a predetermined time sequence.

A voltage measurement device 10 is connected between each of the statorterminals K1, K2, K3 of the induction machine 2, at whose output theterminal voltages u_(K12), u_(K23), u_(K31) which occur in each casebetween the relevant two stator terminals K1, K2, K3 are produced. As analternative to this, the voltages between a stator terminal K1, K2, K3and a neutral conductor, which is not shown in the figure, can also ineach case be measured and used to derive the terminal voltages u_(K12),u_(K23), u_(K31).

The outputs of the voltage measurement devices 10 are connected to acomputation device 12 in which the analog voltage signals u_(K12),u_(K23), u_(K31), which are, for example, present continuously at theinput, are processed further. The computation device 12 contains a firstcomputation unit 14 in which the terminal voltages u_(K12), u_(K23),u_(K31), which are present in the form of analog measured value signals,are subjected to a coordinate transformation in the process of which thecomponents u_(sx) and u_(sy) of the space vector u_(s) ^(<) of theinduced stator voltage and, from this, the angle γ of the space vectoru_(s) ^(<) of this stator voltage, are calculated. The values obtainedin this way for the angle γ of the space vector u_(s) ^(<) are writtencontinuously to a memory 16.

The memory 16 is followed by a second computation unit 18, in which theangles γ stored in the memory 16 are read and are used to calculate therotation speed n of the induction machine 2. The values for the angle γwritten to the memory 16 are in this case processed further in thesecond computation unit 18 only in a time period in which it is certainthat there is no current flowing in the stator of the induction machine2. The read process and computation process in the second computationunit 18 are in this case initialized by the control device 8, in whichthe program routine for the measurement sequence is stored. A controlsignal which corresponds to the rotation speed n is produced at theoutput of the computation device 12 and is passed to one input of thecontrol device 8, where it is evaluated in order to control theinduction machine 2.

The second computation unit 18 is thus initialized only in a time periodin which it is certain that no stator currents i₁, i₂, i₃ are flowing inthe phases L1, L2, L3.

In the graph in FIG. 2, it can be seen that no current is flowing in thestator of the induction machine in a time period Δt. Thus, all thestator currents i₁, i₂, i₃ are equal to zero in this time period Δt.

The situation where no current flows in the stator is now produced firstof all by not passing any trigger pulses to the thyristors 6 (FIG. 1).This leads to initial extinguishing of the current in one of the threestator windings, in the example the current i₃ in the phase L3 at thetime to (initial extinguishing phase). The currents i₁, i₂ in the tworemaining windings or phases L1, L2 are then extinguished at the timet₁, so that no current is flowing in the stator in the time period Δtbetween t₁ and t₂, and the evaluation of the terminal voltage u_(K12),u_(K23), u_(K31) can start.

The stator voltage induced at the terminals of the electrical machine,in the stator coordinate system and when no current is flowing in thestator, is given by: $\begin{matrix}{u_{s}^{\angle} = \quad {{L_{h} \cdot \frac{}{t}}\left( {i_{R}^{\angle} \cdot ^{j \cdot \mathrm{\Upsilon}}} \right)}} \\{u_{s}^{\angle} - \quad {{space}\quad {vector}\quad {of}\quad {the}\quad {stator}\quad {voltage}}} \\{L_{h} - \quad {{main}\quad {inductance}\quad {of}\quad {the}\quad {machine}}} \\{i_{R}^{\angle} - \quad {{space}\quad {vector}\quad {of}\quad {the}\quad {rotor}\quad {current}}} \\{\gamma - \quad {{rotation}\quad {angle}\quad {of}\quad {the}\quad {rotor}\quad {current}\quad {space}\quad {vector}\quad {with}}} \\{\quad {{respect}\quad {to}\quad {the}\quad {stator}\quad {coordinate}\quad {system}}}\end{matrix}$

The following expression is obtained by differentiation:${{u_{s}^{\angle} = {L_{h} \cdot \left( {{^{j \cdot \mathrm{\Upsilon}} \cdot \frac{i_{R}^{\angle}}{t}} + {j \cdot \omega \cdot ^{j \cdot \mathrm{\Upsilon}} \cdot i_{R}^{\angle}}} \right)}}\omega - {{Electrical}\quad {angular}\quad {velocity}\quad {of}\quad {the}\quad {rotor}}},{{{where}\quad \omega} = \frac{\gamma}{t}}$

Since the rate of change of the decaying rotor direct current isnegligibly small in comparison to the change resulting from therotation, the first summand in the bracket in the above equation can beignored, resulting in:

u _(s) ^(<) ≡j·L _(h) ·ω·e ^(j·γ) ·i _(R) ^(<)

It follows from this that the angle between the stator voltage spacevector u_(s) ^(<) and the rotor current space vector i_(R) ^(<) relatedto the stator is constant, and that the frequency of the inducedterminal voltage u_(K12), u_(K23), u_(K31) corresponds to the electricalangular velocity of the rotor. FIG. 3 shows the waveform of the terminalvoltages u_(K12), u_(K23), u_(K31).

The position of the stator voltage space vector u_(s) ^(<) is nowdetermined from the three measured terminal voltages u_(K12), u_(K23),u_(K31) by means of a coordinate transformation, which is known per se:$\begin{matrix}{u_{s}^{\angle} = \quad {{u_{SX} + {j \cdot u_{SY}}} = {\left( {{\frac{2}{3} \cdot u_{K12}} - {\frac{1}{3} \cdot u_{K23}} - \frac{1}{3}} \right) + {j \cdot}}}} \\{\quad \left( {{\frac{1}{\sqrt{3}} \cdot u_{K23}} - {\frac{1}{\sqrt{3}} \cdot u_{K31}}} \right)} \\{u_{SX} - \quad {x\text{-}{component}\quad {of}\quad {the}\quad {stator}\quad {voltage}\quad {space}\quad {vector}\quad u_{s}^{\angle}}} \\{u_{SY} - \quad {y\text{-}{component}\quad {of}\quad {the}\quad {stator}\quad {voltage}\quad {space}\quad {vector}\quad u_{s}^{\angle}}} \\{u_{K12},u_{K23},{u_{K31} - \quad {{voltages}\quad {which}\quad {can}\quad {be}\quad {measured}\quad {between}\quad {the}}}} \\{\quad {{{stator}\quad {terminals}\quad {K1}},{{K2}\quad {and}\quad {K3}}}}\end{matrix}$

The physical orientation (angle) γ of the stator voltage space vectoru_(s) ^(<) is obtained from the known relationship:$\gamma = {{\arg \left( u_{s}^{\angle} \right)} = {\arctan \left( \frac{u_{SY}}{U_{SX}} \right)}}$

A number of measured values of the physical position of the statorvoltage space vector u_(s) ² are obtained by determining the terminalvoltages u_(K12), u_(K23), u_(K31) and calculating the angle γ withinthe time period Δt within which no current is flowing in the stator, anumber of times. These measured values are shown plotted with respect totime in the graph in FIG. 4. Ideally, at a constant speed, these valuesproduce a straight line G, whose gradient α corresponds directly to thesought electrical angular velocity ω of the rotor.

In order to obtain a reliable measured value for the electrical rotorangular velocity and to minimize the influence of measurement errors,the gradient is determined with the aid of a comparison straight line,which can be obtained from the recorded angle values by appropriatemathematical methods, preferably by minimizing the squares of theerrors.

The mechanical rotor angular velocity is now obtained from theelectrical rotor angular velocity simply by dividing by the known numberof pole pairs p in the induction machine.

In order to keep the influence on the drive of the time period duringwhich no current is flowing low, the sets of active devices must beretriggered such that the torque and stator currents respondapproximately as if no rotation speed measurement had been carried out.

According to FIG. 2, this can be done by increasing the triggering timet₃ of the last triggering (but which was not carried out) of theinitially extinguishing active device arrangement V3 by half the powersupply system period T (=180°) and placing it at the time t₂=t₃+T/2. Inorder to obtain a stator current flow after this retriggering, theretriggering of the active device arrangement V1 which follows theinitially extinguishing active device arrangement V3 in the power supplysystem rotation direction also being placed at the triggering time t₂which results from this. When the current flow starts in response to thefirst retriggering, the actual rotation speed measurement is terminated,since the induced terminal voltages are once again governed by thestator current flowing and thus do not include any measurement signalcontaining the rotor angular velocity.

Thus, depending on the type of electrical machine and the loadconditions, approximately one-third of a power supply system period isavailable for the rotation speed measurement. This is completelysufficient for the described method.

The remaining active device arrangement V2 is triggered with a delay ofone-sixth of the power supply system period (=60°) with respect to theinitially extinguishing active device set at the time t₄, resulting inthe recreation of the normal cycle of active device triggeringprocesses.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A method for measuring rotation speed of aninduction machine whose stator is connected, via a controllable ACcontroller having active device arrangements, to an AC power supplysystem, comprising: controlling the active device arrangements todisconnect the stator from the AC power supply system for at least onepredetermined time period (Δt), which is less than half of a period (T)of a voltage of the AC power supply system, by controlling the activedevice arrangements; measuring in the time period (Δt), a voltage whichis induced in the stator by rotary movement of a rotor and using themeasured voltage to determine components of a stator voltage spacevector; and determining a rotation frequency of the stator voltage spacevector from the measured voltage, and deriving the rotation speed of theinduction machine therefrom.
 2. The method as claimed in claim 1,wherein the rotation speed of the induction machine is measured in anumber of successive time periods whose time interval is 5 to 15 timesthe period (T) of the AC power supply system voltage.
 3. The method asclaimed in claim 1, wherein thyristors are used as the active devicearrangements, and wherein the induction machine is disconnected from theAC power supply system by omitting trigger signals which are required totrigger the thyristors.
 4. The method as claimed in claim 3, furthercomprising: delaying, in order to resume power supply system operationof the induction machine in the case of a three-phase induction machine,a first trigger signal for a first-opened first active devicearrangement in one phase by a multiple of half the period (T) withrespect to a last trigger signal for the first active devicearrangement; triggering at the same time as triggering of this firstactive device arrangement is renewed, a second active device arrangementthat is triggered subsequently in normal operation; and triggering thethird active device arrangement one-sixth of the power supply systemperiod (T) after the triggering of the first active device arrangement,with the trigger signal sequence which was present before thedisconnection then being reproduced.
 5. The method as claimed in claim1, further comprising: measuring in a polyphase AC power supply system,a terminal voltage which is in each case induced in the stator windingsbetween the stator terminals.
 6. The method as claimed in claim 5,further comprising: calculating an angle of the space vector of theinduced stator voltage, in each case, from the measured values of theterminal voltage.
 7. The method as claimed in claim 6, whereinassociated angles of the stator voltage space vector are calculated aplurality of times, and wherein a straight line, from whose gradient therotation speed of the induction machine is determined, is calculated forthe associated angles which occur within the time period (Δt) and whichare each associated with specific times.
 8. The method of claim 1,wherein the AC power supply system is single phase.
 9. The method ofclaim 1, wherein the AC power supply system is polyphase.
 10. A devicefor determining rotation speed of an induction machine whose stator isconnected via an AC controller to an AC power supply system, comprising:a control device for controlling the AC controller, and fordisconnecting the stator from the AC power supply system for apredetermined time period (Δt), which is shorter than half a period (T)of a voltage of the AC power supply system, by controlling active devicearrangements of the AC controller; a voltage measurement device formeasuring at least one terminal voltage which is induced in the statorby rotary movement of a rotor in the time period (Δt); and a computationdevice for calculating a frequency of the measured terminal voltage andfor calculating the rotation speed of the induction machine from thecalculated frequency, wherein a control signal for the control device ispresent at one output of the computation device, the control signalbeing derived from the rotation speed and being passed to the controldevice.
 11. The device as claimed in claim 10, wherein the active devicearrangements include thyristors.
 12. The device as claimed in claim 10,wherein the AC power supply system is a polyphase AC power supplysystem, and a voltage measurement device is arranged between each of thestator terminals.
 13. The device as claimed in claim 10, wherein thecomputation device includes a first computation unit for calculating anangle of a space vector of an induced stator voltage from respectivemeasured values, and a memory for storing the calculated angles.
 14. Thedevice as claimed in claim 13, wherein the computation device includes asecond computation unit for calculating a gradient of a straight line,which is formed by angle and time value pairs stored in the memory, andfor determining the frequency of the space vector of the induced statorvoltage and the rotation speed of the induction machine.
 15. Theapparatus of claim 10, wherein the AC power supply system is singlephase.
 16. The apparatus of claim 10, wherein the AC power supply systemis polyphase.
 17. The device as claimed in claim 11, wherein the ACpower supply system is a polyphase AC power supply system, and a voltagemeasurement device is arranged between each of the stator terminals. 18.The device as claimed in claim 17, wherein the computation deviceincludes a first computation unit for calculating an angle of a spacevector of an induced stator voltage from respective measured values, anda memory for storing the calculated angles.
 19. The device as claimed inclaim 18, wherein the computation device includes a second computationunit for calculating a gradient of a straight line, which is formed byangle and time value pairs stored in the memory, and for determining thefrequency of the space vector of the induced stator voltage and therotation speed of the induction machine.